The concept of producing plied yarns using the false-twist, self-twist phenomenon is now rather well known in the art. Documents in which the general principles of false-twisting and self-twisting are described include the following:
"Self-Twist Yarn," D. E. Henshaw, Merrow Publishing Co., Ltd., Watford, Herts, England, 1971 and PA1 Pat. Nos.: RE 27,717, Breen et al; 3,225,533, Henshaw; 3,306,023, Henshaw et al; 3,353,344, Clendening, Jr.; 3,434,275, Backer et al; 3,443,370, Walls; 3,507,108, Yoshimura et al; 3,717,988, Walls; 3,775,955, Shah; 3,940,917, Strachan.
Reference is also made to U.S. patent application Ser. No. 755,671, filed in the names of the present inventors on Dec. 30, 1976, now U.S. Pat. No. 4,074,511, which discloses apparatus usable with the present invention.
For purposes of convenience, some general comments concerning false-twisted and self-twisted plied yarn will be described. It is possible to form a plied yarn by false-twisting two or more singles yarn strands, attaching the strands to each other and then permitting the strands to wrap about each other using the release of forces stored by the false-twisting to accomplishing the plying, hence the term "self-twist." The false-twisting itself, in simplified form, involves holding spaced points of a yarn strand and twisting the strand in one direction at a point intermediate the held points, e.g., the center. This produces twists on one side of the center in one direction and on the other side of the center in the opposite direction. The center of the twisted strand constitutes a point of twist reversal and is called a "node." Clearly, energy is stored in the strand in the twisting step. When two strands similarly false-twisted in the same direction are brought together in side-by-side juxtaposition with their ends held and permitted to act against or with each other by releasing a central node, the stored forces cause the strands to ply, i.e., to wrap around each other spontaneously. The process is enhanced and the product made more stable if the nodes of the two strands are aligned and are joined or locked together before release and plying.
As will be recognized, the torque or twist force exerted by each strand is roughly proportional to the amount of false twist inserted therein and that such force decreases as the strands ply. The plying step itself therefore continues until the stored twist forces in each strand decrease to a point at which the remaining twist forces are exactly counterbalanced by the resistance to further twisting in the plied yarn. Thus, if one begins with individual strands and then false twists the strands and plies them, each strand will end up, in the plied yarn, with some degree of false-twist which can be thought of as some remaining stored potential energy, the force exerted thereby being too small to cause further ply twisting against opposing frictional and reverse direction torque forces in the plied yarn. In a stable plied yarn formed in this fashion, the amount of singles twist always is greater than the amount of ply twist.
Generally speaking, this remaining stored force or energy may not be particularly disadvantageous, depending upon the type of fabric to be produced from the plied yarn. However, when the yarn is to be used to produce certain products such as cut pile carpet, the relationship of the remaining twist in the singles yarn to the amount of ply twist becomes highly significant because of the appearance of the product produced therefrom. In particular, it has been found highly desirable to increase the ratio of ply twist to singles twist in carpet yarns, and that such increase has the desirable effect of eliminating adverse conditions in the finished carpet such as lack of retention of twist in the cut pile tuft tips.